Heretofore, in multicarrier transmission systems, a radio communication system has been proposed whereby subcarriers are divided into blocks and adaptive modulation is performed for each grouped plurality of subcarriers. With this kind of system, by performing adaptive modulation for each block composed of a plurality of subcarriers rather than on a subcarrier-by-subcarrier basis, feedback information (SNR or similar channel quality information) from a receiving apparatus can be provided in block units, as compared with the case where adaptive modulation is performed on a subcarrier-by-subcarrier basis, and feedback information can be reduced proportionally.
Also, when adaptive modulation parameters (modulation scheme and coding scheme) are reported to a receiving apparatus, it is not necessary to report the modulation scheme and coding scheme of all subcarriers, but instead this information can be reported on a block-by-block basis, enabling the control channel transmission rate to be lowered.
With this kind of radio communication system, the size (frequency band) of a block is fixed so as to be a value at which channel fluctuations within each block can be regarded as constant, and system operation is carried out using this fixed block size (see, for example, the document “Frame Configuration and Control Scheme in MC-CDM Systems with Frequency Scheduling,” Hara et al, Technical Report OF IEICE, RCS 2002-130, pp. 67–72, July 2002).
With the conventional radio communication system shown in the above documentation, it is assumed that channel fluctuation within a block (segment) is constant. However, in an actual radio communication system, as the delay time of a delayed waveform increases, so does the amount of intra-block channel fluctuation, and there are cases where the assumption of regarding intra-block channel fluctuation as constant does not hold. Also, even in cases where there is a delayed waveform with a long delay time, if an attempt is made to make intra-block channel fluctuation constant, the block size must be made smaller, and when this is done, there is a problem in that it becomes difficult to sufficiently reduce the amount of information necessary for a control channel even if subcarriers are blocked.
FIG. 1 is a schematic diagram showing an example of conventional block assignment. When the block size (number of subcarriers in a block) is fixed as in the example of the conventional art shown in FIG. 1, due to the occurrence of a state in which a plurality of blocks are assigned successively on the frequency axis to the same user C, the number of blocks may become unnecessarily large for the actual propagation environment. In this case, there is a problem of the amount of control information becoming unnecessarily large due to the fact that control information (modulation scheme, coding scheme, and so forth) is transmitted to the receiving apparatus on a block-by-block basis.